U.S. Pat. No. 4,277,635, issued July 7, 1981 describes a process for recovering and concentrating ethanol from aqueous solutions containing the ethanol at a lower concentration. For example, the method can be employed, using single or multiple concentration stages, as required, to produce a concentrated ethanol product of 80-95% ethanol, or greater, from an aqueous fermentation beer containing from 1-15% ethanol. The aqueous ethanol feed to be concentrated is passed through a bed of a crystalline silica polymorph capable of selectively adsorbing ethanol from aqueous solutions thereof. Silicalite is indicated as a preferred adsorbent for this purpose. The ethanol is selectively adsorbed in the molecular pore spaces of the porous granules containing the molecular sieve adsorbent. At the conclusion of the adsorption cycle, the disclosure of U.S. Pat. No. 4,277,635 indicates that the bed should be drained to remove excess feed which will be present in the void spaces between the adsorbent granules as well as in the head space above the granular bed. After drainage of the excess liquid, desorption of the ethanol is obtained by passing a heated inert gas, such as carbon dioxide, through the bed. As the passage of the heated gas continues, the granules are heated and the ethanol is evolved as a gas, which is carried out with the carbon dioxide. The ethanol is recovered from the effluent gas by condensation to a liquid.
The ethanol desorption procedure described above has been found to be relatively inefficient. The molecularly adsorbed ethanol is contained within the pore spaces of the adsorbent granules, and the granules are relatively poor conductors of heat. The heat from the carrier gas, or other auxillary heating means if one is used, therefore takes a considerable time to heat the interior portions of the granules to temperatures at which the adsorbed ethanol is vaporized. Further, the rate at which the ethanol is evolved from the granules depends on the rate of diffusion through the capillary passages of the granules, which is a rate limited process. A related disadvantage of the desorption procedure using a heated carrier gas is that the desorption is incomplete. The ethanol remaining molecularly adsorbed within the granules therefore reduces the capacity of the granules for the next adsorption cycle. This means in practice that a larger amount of adsorbent must be used than the theoretical minimum based on the adsorption capacity of the granules, and this increases the cost of the recovery process. Also it has been found that the carrier gas tends to remain within the pore spaces of the granules, being difficult to remove therefrom, and that this residual gas within the porous granules also reduces the capacity of the granules on the next adsorption cycle.